The present invention relates to imaging systems for medical endoscopy, in general and to endoscopic imaging systems for fluorescence and reflectance endoscopy, in particular.
One common diagnostic technique used by physicians to detect diseases within a body cavity of a patient is white light optical fiber endoscopy. With this technique, white light is directed into the body cavity via a non-coherent fiber-optic illumination guide of an endoscope. The light illuminates the tissue under examination and the reflected illumination light is gathered and transmitted through a coherent fiber-optic imaging guide of the endoscope. The image formed by the reflected white light at the end of the imaging guide may be viewed directly through the endoscope eyepiece or may be imaged by a color video camera connected to the eyepiece. Images transduced by the camera are then typically transmitted to an image processing/storage device and to a video monitor where they can be viewed by the physician.
To aid physicians performing endoscopy in detecting the presence of cancerous or pre-cancerous tissue, the differences in the autofluorescence (also referred to as native fluorescence) spectrum of normal and abnormal tissue can be exploited. In fluorescence optical fiber endoscopy, a fluorescence excitation light is delivered into the body cavity via the illumination guide of the endoscope. The wavelengths of this light are matched to the absorption spectrum of the naturally occurring fluorescing molecules (or fluorophores) present in the tissue (i.e., to blue light). The fluorescence excitation light causes the tissue in the body cavity to fluoresce with a green and red emission spectrum and the resulting light is collected and transmitted through the optical fiber imaging guide of the endoscope. The resulting image is transduced by a camera that filters out any reflected blue light and divides the autofluorescence into two broad (green and red) spectral bands. The image formed by the light in each spectral band is projected onto a separate intensified CCD (ICCD) transducer and the resulting signal is fed into a control center for processing, storage and, finally, for display on a video monitor. The difference in the autofluorescence emission spectrum of normal and abnormal tissue is presented as a difference in color on the video monitor.
Systems for fluorescence fiber endoscopy are fully described in U.S. Pat. Nos. 5,507,287; 5,590,660, 5,647,368 and 4,786,813 that are assigned to Xillix Technologies Corp. of Richmond, BC, Canada, the assignee of the present invention, and are sold by Xllix as the Xillix(copyright) LIFE-Lung Fluorescence Endoscopy System(copyright) (the xe2x80x9cLIFE-Lung Systemxe2x80x9d). Multi-center clinical trials have shown that by using the Xllix LFE-Lung System as an adjunct to white light endoscopy, the physician""s sensitivity in detecting moderate dysplasia, or worse, is 2.71 times greater than the sensitivity of a physician using white light endoscopy alone.
The current LIFE-Lung System has a number of limitations, however. First, the current embodiment of the system requires the physician to manually adjust the gain of the system (i.e., to increase and decrease the camera""s sensitivity to the tissue autofluorescence). This is a cumbersome task for the physician to perform, when he/she is simultaneously trying to maneuver the endoscope in the patient. Although automatic gain control circuits for video systems are widely available, they do not provide adequate gain control for the complex scene conditions encountered in imaging autofluorescence with ICCDs. If, for example, the average brightness of an image is increased to an acceptable level, there may be bright spots that can damage the ICCDs. Similarly, if the peak brightness of an image is reduced to prevent localized image saturation, the remainder of the image may become too dark to be recognizable. Furthermore, commonly available average and peak-based automatic gain control circuits do not provide images with a good dynamic range under a variety of viewing conditions, i.e. with an optimized contrast. In endoscopy, these viewing conditions include situations whereby the range of fluorescence light intensities are greater than the dynamic range of ICCDs and the image scenes vary from complex structures (i.e. lots of intensity variations) to flat structures (i.e. homogeneous).
A further complication with the use of an automatic gain control circuit arises due to the fact that the gain relationship between the two channels (green and red) of the imaging system must follow a defined function. If the gain of each channel is varied independently, the colors in the resulting video image will not consistently reflect the spectral differences in the autofluorescence of the tissue.
A second limitation of the current LIFE-Lung System becomes evident when a physician wishes to switch between white light (reflectance) and fluorescence imaging modes. With the current system, the physician must switch light sources and cameras manually (i.e., from a white light illumination source to a fluorescence excitation light source and from an RGB color video camera to the fluorescence camera). One technique for addressing this time consuming process is to have all light sources and cameras connected to the endoscope simultaneously and to utilize a mode switching mechanism to switch from one imaging mode to the other. However, some precaution must be taken in the implementation of a switching mechanism since the ICCDs can be damaged if they are subjected to the bright, reflected illumination light. Care must be taken to ensure that the ICCDs are not energized unless the appropriate illumination conditions exist.
A third limitation of the current LIFE-Lung System is that a physician viewing the image displayed by the system has no way of objectively quantifying the extent of abnormality exhibited by the tissue under examination. The effective use of the system is dependent on such subjective factors as the physician""s ability to distinguish color and his/her ability to interpret this color information in the context of other image features. A means to objectively quantify the difference in the autofluorescence spectra of normal and abnormal tissue, or even an additional means to subjectively differentiate these tissues based on their difference in autofluorescence spectra could improve the clinical usability of this system. This can be accomplished using computational techniques using the spectral information of the emitted fluorescence and displaying the results on the monitor together with the images.
In summary, the operation of current fluorescence endoscopy systems may be significantly improved by:
a) an automatic gain control circuit that will optimally adjust the brightness of autofluorescence images and that will maintain a defined relationship between the two channels of the imaging system;
b) a mechanism that allows rapid switching between white light and fluorescence imaging modes, while preventing the accidental exposure of energized ICCDs to damaging light intensities; and
c) a means of utilizing the differences in the autofluorescence emission spectra of normal to abnormal tissue to objectively quantify the degree of abnormality of the tissue.
The present invention is an imaging system for white light and fluorescence endoscopy that includes a particular automatic gain control (AGC) circuit in the fluorescence imaging mode. The AGC circuit adjusts the gain of the imaging system by adjusting the gain of two high sensitivity imaging devices such as image intensified CCD (ICCDs) transducers in a fluorescence camera head and by adjusting the light intensity of the excitation light source. The video signals from a pair channels (the xe2x80x9cgreenxe2x80x9d and xe2x80x9credxe2x80x9d channel) of a fluorescence camera are supplied to a set of counters. The counters, consisting of counters connected to a clocking oscillator, measure the length of time each video signal has a magnitude that exceeds a reference threshold that is individually set for each counter. Thus, by appropriately arranging the threshold levels, the outputs of the counters can be made to indicate the distribution of video signal amplitudes in one or more video fields. Based upon the outputs of the counters, a decision tree algorithm determines if the gain of the imaging system or the light source intensity should be increased or decreased. A gain control equation determines the appropriate value of light source intensity change and maps the resulting imaging system gain increase or decrease to an individual gain change for each ICCD transducer such that the relative gain between the two channels remains the same.
The present invention also includes a mode switching mechanism that allows for convenient switching between white light and fluorescence endoscopy imaging modes. The implementation of mode switching implies that white light and fluorescence light sources and cameras are connected to the endoscope simultaneously and that the appropriate combination of camera and light source are activated when switching modes. This requires a two-part mode switching mechanism: one switching the cameras and one switching the light sources. The camera mode switching mechanism consists of a light directing mechanism such as a mirror that is movable between a first position, where the image from the endoscope is reflected towards an RGB video camera head, and a second position, where the image from the endoscope is allowed to pass to the fluorescence camera head. When a physician uses the mode switch to change from white light imaging to fluorescence imaging or vice versa, a pair of proximity switches provide signals to the system control center, which monitors the position of the mirror, to ensure that the ICCDs are not energized until the appropriate light source has been selected. The light source mode switching mechanism consists of a filter driver that positions blue, fluorescence excitation filters or white light filters in an illumination light path that extends between the light source and an endoscope.
The present invention also provides a means of objectively quantifying the spectral differences between normal and abnormal tissue by using the relative brightness of autofluorescence in the spectral bands being imaged (green and red). A portion of the autofluorescence image is analyzed and the numerical value defined by a particular mathematical function such as the ratio of the image brightnesses of the two wavebands is displayed for the physician.